3D Mechanochromic Sensors from Core-Shell Cholesteric Liquid Crystal Elastomers

Cholesteric liquid crystal elastomers (CLCEs) hold great promise for mechanochromic applications in anti-counterfeiting,[1] smart textiles,[2] and soft robotics,[3] thanks to their structural color and elasticity. While CLCEs have been printed via direct-ink-writing (DIW) to fabricate free-standing films, three-dimensional (3D) structures have not been realized yet due to the opposing rheological properties necessary for cholesteric alignment and multilayer stacking. Here, we present a methodology to print 3D CLCE-silicone structures with a core material derived from a low-viscosity cholesteric liquid crystal (CLC) ink and a shell material prepared from a yield-stress silicone ink. The cholesteric phase forms in under one minute, without requiring an annealing step, and the silicone shell provides encapsulation and support to the CLC core, allowing for layer-by-layer printing without requiring in-situ UV crosslinking. Both inks are prepared without the use of organic solvent and are printed at room temperature, which enables facile scaling up of CLCE fabrication in terms of dimension and speed. In addition to providing support for the uncured CLC, the silicone shell enhances the mechanochromic sensitivity and increases the fracture strain of the cured CLCE core. As a demonstration of the capabilities of the inks and printing method, monostable and bistable thin-shell domes are fabricated. The CLCE-silicone domes can invert without cracking due to the softer and more elastic silicone shell. The inverted domes can be viewed from the top or the bottom to witness a red shift or blue shift in color due to compressive or tensile stresses, respectively. When bistable domes of different heights are arranged in an array and inverted, the domes can be sequentially snapped through back to their stable base state by uniaxial stretching, thereby functioning as a mechanochromic sensor with memory. Our findings regarding the inks and procedure presented enable the fabrication of 3D mechanochromic sensors, thus expanding the realm of potential applications for CLCEs.

[1] M. Liu, J. Fu, S. Yang, Y. Wang, L. Jin, S. H. Nah, Y. Gao, Y. Ning, C. B. Murray, S. Yang, Adv. Mater. 2023, 35, 2207985.
[2] Y. Geng, J. P. F. Lagerwall, Adv. Sci. 2023, 10, 2301414.
[3] S.-U. Kim, Y.-J. Lee, J. Liu, D. S. Kim, H. Wang, S. Yang, Nat. Mater. 2022, 21, 41.